Auxin increases elastic wall‐properties in rye coleoptiles: implications for the mechanism of wall loosening

Abstract

Auxin‐induced changes of wall‐rheological properties during different growth rates of rye coleoptile segments (Secale cereale L.) were investigated. In addition, changes of osmotic concentration and turgor pressure were measured. Decrease of turgor and of osmotic concentration followed a synchronous time course. Auxin‐incubated segments exhibited a faster decrease and eventually lower values of both parameters. Creep test extensibility measurements demonstrate that apparent plastic as well as elastic extensibility of distilled‐water‐incubated segments strongly decreased during 24 h. In auxin‐incubated segments apparent plastic as well as elastic extensibilities were strongly increased, even in the absence of growth due to insufficient turgor pressure. The increasing effect of auxin on elastic wall properties is also reflected by an increase in relative reversible length (part of segment length by which segments shrink after freezing/thawing as referred to total length) and a complementary decrease of relative irreversible length (remaining length after turgor elimination as referred to turgid length); again the effects were independent of growth rate and turgor pressure. Cellulose synthesis inhibition of approx. 80% by dichlorobenzonitrile (DCB) had no significant effect either on growth or on wall‐rheological properties. Independent of whether the changed rheological wall behaviour of auxin‐incubated segments is causally related to the mechanism of auxin‐induced wall loosening, it indicates changes of wall polymer properties and/or interactions which are conserved when no actual length increase occurs due to insufficient turgor pressure. The results suggest that IAA‐induced wall loosening may be primarily mediated by cell wall changes other than cleavage of covalent, load‐bearing bonds as hypothesized in various wall loosening models.